I also have a ipw3945 card, but it does NOT share IRQ with nvidia (nothing does), but I still have flickers...

That might just be a coincidence (post hoc ergo propter hoc selection), honestly I did not witness that behaviour often. 90% of the times I just have flickers until I restart X.

That's enough to spoil the fun, though.

BTW: i upgraded the BIOS to the latest ASUS release which has also an "upgraded" video BIOS, having a more recent release date, but a smaller release number. No joy with the flickers, but strange enough.

But...does this happen only when you have the second monitor on?
Or only adding the Twinview's options in xorg.conf?

And...just to add more info about this bug:
- It happens from time to time.
- It's also present with xorg 7.2(I reproduced it).
- It seems not having any relationship with the ipw card(My ipw doesn't share the IRQ with the nvidia).
- It's present with the newer kernels(I reroduced it with 2.6.20-rc4, with 2.6.21-rc1, with 2.6.20-beyond).
- It seems not having any relationship with CPU frequency scaling.
- It's related to Composite Managers(Beryl or Compiz): switching them off, it's not reproducible.
- It's making me become mad!

EDIT:
Can someone confirm it's not reproducible with Xgl instead of NvidiaGLX(AiGLX)?

A small tip I can do to stop the black flash(only temporally) is to change the screen frequency from "Auto" to "60Hz"(my video frequency) as shown in this image:

I am experiencing similar flickers on an Asus F3Jc (Core 2 Duo / GForceGo 7300), but with some important differences. I'm trying to get a good picture on an external Samsung SyncMaster 204B.

My experiences (yes, I have spent a _lot_ of time trying to work this out):
1. I have no problems with the built-in DFP. Possibly because of the low resolution (1280x800).
2. I have no problems with VGA output in high resolutions (1600x1200), nor with DVI in 1280x1024 or less.
3. I _only_ experience the flickering when using 1600x1200 over DVI.
3 1/2. The flickering is also present in Windows (Vista), but even worse. Also only in 1600x1200.
4. Another computer (with a GeForce 6200) has no problems at all with the same monitor (and same DVI cable) in 1600x1200.
5. I don't get any XID logs in dmesg, nor have I had any real crashes.
6. The PerfLevelSrc trick doesn't do anything for me (grrrr!)
7. The flickering usually starts when there's some activity, for example starting mozilla on the external screen, then it gets worse and worse until the screen finally goes completely black.
8. _sometimes_ there are random flickering pixels in dark areas between the flickering.
9. I've tried the debian stock 2.6.18-4-686 kernel and 8776 drivers, and a custom 2.6.20.1 kernel, 9755 and 9746 drivers. I have also tried many different combinations of Xorg.conf options and kernel parameters.

I'm a bit pissed off, because I bought this laptop because it had DVI. Now I have to use blurry VGA until this can be solved.

It really helps (in my case I have sometimes the Xid and black screen or the horizontal screen shaking) - great!

But it seems as if this option causes the GPU not to change the frequency anymore. nvclock -s returns constant values. Without the option the GPU frequency goes down when there is no GPU activity. (Which is okay because I had the horizontal screen shaking exactly when the frequency changes).

UPDATE:
I added the option to: /etc/modprobe.d/nvidia and the screen locked at the loginscreen and only a hard shutdown and removing the option solved the problem, and I have gone back to the nv driver.